Many disc drives use magnetic heads and recorded servo code in a track following mode for keeping the heads track centered during read/write operations. The electromagnetic transducers of these magnetic heads comprise a magnetic circuit having a coil wound thereon. These magnetic circuits vary in effective magnetic widths due to their design and due to the manufacturing process. This variation is not uniform among the magnetic heads and results in variations in servo gain when they are individually connected in the servo loop. Manual adjustment of servo gain was practiced in some instances in the past to compensate for these magnetic head variations.
More recently, automatic gain control systems have been devised for disc drives for automatically compensating for variations in servo gain. Two such systems, representing the closest art to this invention which is presently known to the applicants, are represented in U.S. Pat. Nos. 4,551,776 and 4,578,723.
U.S. Pat. No. 4,551,776 entitled "Automatic Reference Adjustment for Position Error Signal on Disc File Servo System", periodically recalibrates the position error signal to increase or decrease its magnitude by modifying the gain of a variable gain amplifier in the servo loop.
U.S. Pat. No. 4,578,723 entitled "Head Positioning System With Automatic Gain Control", describes a system with automatic gain control which is stated to be substantially independent of head widths and to limit variations in off track gain between heads. Multiphase radial position error signals derived from position reference information on the disc are used to control the position of a transducing head by means of a head positioning actuator. The gain of a variable gain amplifier responsive to the signals from a particular magnetic head connected in the servo loop, is controlled by a gain function derived by combining the different phase position error signals to provide, at any position of the magnetic head, a measurement of the rate of change of the position error signal per track of displacement. The output of the variable gain amplifier is used to control the position of the magnetic head.
Thus, according to the teachings of the first patent, the position error signal is recalibrated to keep it within limits by modifying the gain of a variable gain amplifier and according to the teachings of the second patent, the rate of change of the position error signal per track of displacement is used to control the gain of a variable gain amplifier.